Method for manufacturing semiconductor device

ABSTRACT

An object is to provide a method for manufacturing a semiconductor device including an oxide semiconductor and having improved electric characteristics. The semiconductor device includes an oxide semiconductor film, a gate electrode overlapping the oxide semiconductor film, and a source electrode and a drain electrode electrically connected to the oxide semiconductor film. The method includes the steps of forming a first insulating film including gallium oxide over and in contact with the oxide semiconductor film; forming a second insulating film over and in contact with the first insulating film; forming a resist mask over the second insulating film; forming a contact hole by performing dry etching on the first insulating film and the second insulating film; removing the resist mask by ashing using oxygen plasma; and forming a wiring electrically connected to at least one of the gate electrode, the source electrode, and the drain electrode through the contact hole.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/156,472, filed Jun. 9, 2011, now allowed, which claims the benefit ofa foreign priority application filed in Japan as Serial No. 2010-139715on Jun. 18, 2010, both of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to a semiconductor device and a method formanufacturing the semiconductor device.

In this specification, a semiconductor device generally means a devicewhich can function by utilizing semiconductor characteristics, andelectrooptic devices, semiconductor circuits, and electronic devices areall semiconductor devices.

BACKGROUND ART

A technique by which a transistor is manufactured using a semiconductorthin film formed over a substrate having an insulating surface has beenattracting attention. The transistor is applied to a wide range ofelectronic devices such as an integrated circuit (IC) or an imagedisplay device (a display device). A silicon-based semiconductormaterial is widely known as a material for a semiconductor thin filmapplicable to a transistor. As another material, an oxide semiconductorhas been attracting attention.

For example, a transistor whose semiconductor layer used for a channelformation region is formed using an amorphous oxide including indium(In), gallium (Ga), and zinc (Zn) and having an electron carrierconcentration lower than 10¹⁸/cm³ is disclosed (see Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    2006-165528

DISCLOSURE OF INVENTION

However, the electric conductivity of an oxide semiconductor mightchange when deviation from the stoichiometric composition ratio due todeficiency of oxygen or the like occurs, or when hydrogen or waterbehaving as electron donor enters the oxide semiconductor during amanufacturing process of a device. Such phenomena are factors ofvariation in electric characteristics of a semiconductor device such asa transistor including an oxide semiconductor.

In view of the above problem, an object is to provide a method formanufacturing a semiconductor device including an oxide semiconductorand having improved electric characteristics.

Another object is to solve a problem which may arise in the manufactureof the semiconductor device including an oxide semiconductor and havingimproved electric characteristics.

In one embodiment of the present invention, an insulating film includinga constituent element of an oxide semiconductor film (e.g., aninsulating film including gallium oxide) is provided so as to be incontact with the oxide semiconductor film, and direct wet treatment isnot used in an etching step of the insulating film, whereby a favorablecondition is maintained at an interface between the oxide semiconductorfilm and the insulating film. More concretely, the following structurecan be given as example.

One embodiment of the disclosed invention is a method for manufacturinga semiconductor device including an oxide semiconductor film, a gateelectrode, and a source electrode and a drain electrode that areelectrically connected to the oxide semiconductor film. The methodincludes the steps of forming the oxide semiconductor film over the gateelectrode; forming a first insulating film including gallium oxide overand in contact with the oxide semiconductor film, the source electrode,and the drain electrode; forming a second insulating film over and incontact with the first insulating film; forming a resist mask over thesecond insulating film; forming a contact hole by performing dry etchingon the first insulating film and the second insulating film; removingthe resist mask by ashing using oxygen plasma; and forming a wiringelectrically connected to at least one of the gate electrode, the sourceelectrode, and the drain electrode through the contact hole.

Another embodiment of the disclosed invention is a method formanufacturing a semiconductor device including an oxide semiconductorfilm, a gate electrode, and a source electrode and a drain electrodethat are electrically connected to the oxide semiconductor film. Themethod includes the steps of forming a first insulating film includinggallium oxide over and in contact with the oxide semiconductor film, thesource electrode, and the drain electrode; forming the gate electrodeover and in contact with the first insulating film to overlap with theoxide semiconductor film; forming a second insulating film over and incontact with the first insulating film; forming a resist mask over thesecond insulating film; forming a contact hole by performing dry etchingon the first insulating film and the second insulating film; removingthe resist mask by ashing using oxygen plasma; and forming a wiringelectrically connected to at least one of the source electrode and thedrain electrode through the contact hole.

In the above structure, the diameter of the contact hole in the firstinsulating film is preferably smaller than the diameter of the contacthole in the second insulating film. The thickness of the secondinsulating film is preferably larger than the thickness of the firstinsulating film.

Further, a gas including fluorine is preferably used in the dry etching.

In the case where the first insulating film further includes aluminumoxide, a gas including chlorine is preferably used in the dry etching.

In addition, it is preferable that the first insulating film be formedwhile applying heat.

Further, the oxide semiconductor film is preferably an i-type oxidesemiconductor film. Here, an i-type (intrinsic) oxide semiconductorrefers to an oxide semiconductor which is highly purified by removinghydrogen, which is an n-type impurity for an oxide semiconductor, fromthe oxide semiconductor so that impurities that are not main componentsof the oxide semiconductor are included as few as possible, and in whichoxygen deficiency defects are reduced by supplying oxygen. In an i-typeoxide semiconductor film, hydrogen atoms serving as electron donors anddefects due to oxygen deficiency are sufficiently reduced; therefore,degradation of the interface characteristics due to impurities anddefects can be suppressed.

Note that purification of an oxide semiconductor also has an effect ofpreventing variation in electric characteristics of a transistor.Therefore, purification of an oxide semiconductor is extremely effectivefor improvement in transistor characteristics.

According to one embodiment of the present invention, a method formanufacturing a semiconductor device in which a favorable condition ismaintained at an interface between an oxide semiconductor film and aninsulating film can be provided. Thus, a method for manufacturing asemiconductor device having improved electric characteristics can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1E illustrate an example of a manufacturing process of asemiconductor device;

FIGS. 2A to 2C illustrate an example of a manufacturing process of asemiconductor device;

FIGS. 3A to 3E illustrate an example of a manufacturing process of asemiconductor device;

FIGS. 4A, 4B1, and 4B2 illustrate discharge states in an AC sputteringmethod;

FIGS. 5A to 5C illustrate embodiments of a semiconductor device;

FIG. 6 illustrates one embodiment of a semiconductor device;

FIG. 7 illustrates one embodiment of a semiconductor device;

FIG. 8 illustrates one embodiment of a semiconductor device;

FIGS. 9A to 9F illustrate electronic devices;

FIGS. 10A and 10B are SEM images of samples according to Example 1; and

FIGS. 11A and 11B are SEM images of samples according to Example 1.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and an example of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails disclosed herein can be modified in various ways. Therefore, thepresent invention is not construed as being limited to description ofthe embodiments. Note that in the following description, portions thatare common between drawings are denoted by the same reference numerals,and repeated description is omitted.

Note that in this specification the ordinal numbers such as “first” and“second” are used for convenience and do not denote the order of stepsor the stacking order of layers. In addition, the ordinal numbers inthis specification do not denote particular names which specify theinvention.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and oneembodiment of a method for manufacturing the semiconductor device willbe described with reference to FIGS. 1A to 1E, FIGS. 2A to 2C, FIGS. 3Ato 3E, and FIGS. 4A, 4B1, and 4B2.

<Manufacturing Process of Transistor 110>

FIGS. 1A to 1E are cross-sectional views illustrating a manufacturingprocess of a transistor 110 as an example of a method for manufacturinga semiconductor device according to one embodiment of the disclosedinvention. Here, the transistor 110 includes, over a substrate 200, agate electrode 202, an insulating film 204, an oxide semiconductor film206, a source electrode 208 a, a drain electrode 208 b, an insulatingfilm 210, an insulating film 212, and a wiring 216. In the transistorillustrated in FIG. 1E, the insulating film 210, including galliumoxide, is provided in contact with the oxide semiconductor film 206, andthe insulating film 212 is provided over and in contact with theinsulating film 210 including gallium oxide. In addition, the wiring 216is electrically connected to the source electrode 208 a through acontact hole 218 formed in the insulating film 212 and the insulatingfilm 210.

Here, the oxide semiconductor film 206 is preferably an oxidesemiconductor film which is highly purified by sufficiently removingimpurities such as hydrogen or water, or by sufficiently supplyingoxygen. Specifically, the hydrogen concentration of the oxidesemiconductor film 206 is 5×10¹⁹ atoms/cm³ or lower, preferably 5×10¹⁸atoms/cm³ or lower, further preferably 5×10¹⁷ atoms/cm³ or lower, forexample. Note that the hydrogen concentration of the oxide semiconductorfilm 206 is measured by secondary ion mass spectrometry (SIMS). In theoxide semiconductor film 206 which is highly purified by sufficientlyreducing the hydrogen concentration and in which defect levels in theenergy gap due to oxygen deficiency are reduced by sufficientlysupplying oxygen, the carrier concentration is lower than 1×10¹²/cm³,preferably lower than 1×10¹¹/cm³, further preferably lower than1.45×10¹⁰/cm³. For example, the off-state current (here, current permicrometer (μm) of channel width) at room temperature (25° C.) is lowerthan or equal to 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A), preferablylower than or equal to 10 zA. In this manner, by using an i-type oxidesemiconductor, a transistor having favorable electric characteristicscan be obtained.

In many cases, an oxide semiconductor material used for the oxidesemiconductor film 206 includes gallium. Therefore, when the insulatingfilm 204 or the insulating film 210, which is in contact with the oxidesemiconductor film, is formed using a material including gallium oxide,a favorable condition can be maintained at an interface between theoxide semiconductor film and the insulating film. For example, byproviding the oxide semiconductor film in contact with the insulatingfilm including gallium oxide, an accumulation of hydrogen at aninterface between the oxide semiconductor film and the insulating filmcan be reduced. This is because a material including gallium oxide iscompatible with an oxide semiconductor material.

Note that, in the case where an element belonging to the same group as aconstituent element of the oxide semiconductor is used for theinsulating film 204 or the insulating film 210, a similar effect can beobtained. That is, it is also effective to additionally use a materialincluding aluminum oxide or the like for the insulating film 204 or theinsulating film 210. Aluminum oxide tends to resist to waterpenetration; therefore, it is preferable to use a material includingaluminum oxide in the optic of preventing water from entering the oxidesemiconductor film. For example, a material including gallium andaluminum, such as aluminum gallium oxide (or gallium aluminum oxide) maybe used for the insulating film 204 or the insulating film 210. In thiscase, both the effect resulting from inclusion of gallium and the effectresulting from inclusion of aluminum can be obtained, which ispreferable. By providing the oxide semiconductor film in contact with aninsulating film including aluminum gallium oxide, for example, water canbe prevented from entering the oxide semiconductor film and anaccumulation of hydrogen at an interface between the oxide semiconductorfilm and the insulating film can be sufficiently reduced.

An example of the manufacturing process of the transistor 110illustrated in FIG. 1E will be described below with reference to FIGS.1A to 1E.

First, a conductive film for forming the gate electrode (including awiring formed in the same layer as the gate electrode) is formed overthe substrate 200 and then processed, so that the gate electrode 202 isformed. After that, the insulating film 204 is formed so as to cover thegate electrode 202 (see FIG. 1A).

There is no particular limitation on the material or the like of thesubstrate 200 as long as the material has heat resistance enough towithstand at least heat treatment performed later. For example, a glasssubstrate, a ceramic substrate, a quartz substrate, a sapphiresubstrate, or the like can be used as the substrate 200. Alternatively,a single crystal semiconductor substrate or a polycrystallinesemiconductor substrate made of silicon, silicon carbide, or the like, acompound semiconductor substrate made of silicon germanium or the like,an SOI substrate, or the like can be used. Further alternatively, any ofthese substrates provided with a semiconductor element may be used asthe substrate 200.

A flexible substrate may be used as the substrate 200. In the case wherea transistor is provided over a flexible substrate, for example, thetransistor can be directly formed over the flexible substrate.

As the conductive film used for the gate electrode 202, for example, ametal film including an element selected from molybdenum, titanium,tantalum, tungsten, aluminum, copper, neodymium, and scandium; an alloymaterial including any of these elements as a main component; or thelike can be used. The gate electrode 202 may have a single-layerstructure or a stacked-layer structure.

The conductive film can be processed by being etched after a mask havinga desired shape is formed over the conductive film. As the above mask, aresist mask or the like can be used.

The insulating film 204 functions as a gate insulating film of thetransistor 110. The insulating film 204 is formed using, for example, amaterial such as silicon oxide, silicon nitride, silicon oxynitride, orsilicon nitride oxide. Further, the insulating film 204 can be formedusing a material including gallium oxide. The material including galliumoxide may further include aluminum oxide; that is, a material includingaluminum gallium oxide or gallium aluminum oxide, or the like may beused. Here, aluminum gallium oxide refers to a substance that includesaluminum at a content (atomic %) higher than that of gallium, andgallium aluminum oxide refers to a substance that includes gallium at acontent (atomic %) higher than or equal to that of aluminum. A materialhaving a high dielectric constant, such as hafnium oxide, tantalumoxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)),hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added,or hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen isadded may be used. The insulating film 204 can be formed to have asingle-layer structure or a stacked-layer structure using any of theabove materials.

A gallium oxide film used as the insulating film 204 preferably has acomposition of Ga₂O_(3+α) (α>0). It is preferable that a be greater thanor equal to 3.04 and less than or equal to 3.09. Alternatively, analuminum gallium oxide film used as the insulating film 204 preferablyhas a composition of Al_(x)Ga_(2-x)O_(3+α) (1<x<2, α>0). Furtheralternatively, a gallium aluminum oxide film used as the insulating film204 preferably has a composition of Al_(x)Ga_(2-x)O_(3+α) (0<x≦1, α>0)by being doped with oxygen.

In many cases, an oxide semiconductor material used for the oxidesemiconductor film includes gallium. Therefore, in the case where theinsulating film 204 which is in contact with the oxide semiconductorfilm is formed using a material including gallium oxide, a favorablecondition can be maintained at an interface between the oxidesemiconductor film and the insulating film 204. For example, byproviding the oxide semiconductor film in contact with an insulatingfilm including gallium oxide, an accumulation of hydrogen at aninterface between the oxide semiconductor film and the insulating filmcan be reduced. This is because a material including gallium oxide iscompatible with an oxide semiconductor material.

Note that, in the case where an element belonging to the same group as aconstituent element of the oxide semiconductor is used for theinsulating film 204, a similar effect can be obtained. That is, it isalso effective to additionally use a material including aluminum oxideor the like for the insulating film 204. Aluminum oxide tends to resistto water penetration; therefore, it is preferable to use the materialincluding aluminum oxide in the optic of preventing water from enteringthe oxide semiconductor film. For example, a material including galliumand aluminum, such as aluminum gallium oxide (or gallium aluminum oxide)described above may be used for the insulating film 204. In this case,both the effect resulting from inclusion of gallium and the effectresulting from inclusion of aluminum can be obtained, which ispreferable. By providing the oxide semiconductor film in contact with aninsulating film including aluminum gallium oxide, for example, water canbe prevented from entering the oxide semiconductor film and anaccumulation of hydrogen at an interface between the oxide semiconductorfilm and the insulating film can be sufficiently reduced.

The insulating film 204 is preferably formed by a method with whichimpurities such as hydrogen or water do not enter the insulating film204. This is because, when impurities such as hydrogen or water areincluded in the insulating film 204, the impurities such as hydrogen orwater enter the oxide semiconductor film formed later or oxygen in theoxide semiconductor film is extracted by the impurities such as hydrogenor water, so that a back channel of the oxide semiconductor film mighthave lower resistance (have n-type conductivity) and a parasitic channelmight be formed. Therefore, the insulating film 204 is preferably formedso as to include impurities such as hydrogen or water as few aspossible. For example, the insulating film 204 is preferably formed by asputtering method, and a high-purity gas from which impurities such ashydrogen or water are removed is preferably used as a sputtering gasused for film formation.

Examples of the sputtering method include a DC sputtering method inwhich a direct current power source is used, a pulsed DC sputteringmethod in which a direct bias is applied in a pulsed manner, an ACsputtering method, and the like.

Here, a sputtering method using AC discharge will be described withreference to FIGS. 4A, 4B1, and 4B2. In AC discharge, adjacent targetsalternately have a cathode potential and an anode potential. In a periodA shown in FIG. 4A, a target 301 functions as a cathode and a target 302functions as an anode as illustrated in FIG. 4B1. In a period B shown inFIG. 4A, the target 301 functions as an anode and the target 302functions as a cathode as illustrated in FIG. 4B2. The total time of theperiod A and the period B is 20 μsec 50 μsec, and the period A and theperiod B are repeated at a constant frequency. In this manner, by makingthe two adjacent targets function as a cathode and as an anodealternately, stable discharge can be realized. As a result, even when alarge-sized substrate is used, uniform discharge is possible;accordingly, uniform film characteristics can be obtained also in thecase of using the large-sized substrate. Moreover, since the large-sizedsubstrate can be used, productivity can be increased.

For example, in the AC sputtering method, aluminum oxide is used as thetarget 301 and gallium oxide is used as the target 302, whereby agallium aluminum oxide film or an aluminum gallium oxide film can beformed. As the target 301 and the target 302, a gallium oxide target towhich aluminum particles are added may be used. By using a gallium oxidetarget to which an aluminum element is added, conductivity of the targetcan be increased and discharge can be easily performed during the ACsputtering.

Next, oxygen doping treatment is preferably performed on the insulatingfilm 204. Oxygen doping refers to addition of oxygen (which includes atleast one of an oxygen radical, an oxygen atom, and an oxygen ion) to abulk. Note that the term “bulk” is used in order to clarify that oxygenis added not only to a surface of a thin film but also to the inside ofthe thin film. In addition, the oxygen doping includes oxygen plasmadoping in which oxygen that is made to be plasma is added to a bulk.

When the oxygen doping treatment is performed on the insulating film204, a region where the amount of oxygen is larger than that in thestoichiometric composition ratio is formed in the insulating film 204.By providing such a region, oxygen can be supplied to the oxidesemiconductor film and oxygen deficiency defects in the oxidesemiconductor film can be reduced.

Note that in the case where an oxide semiconductor without defects(oxygen deficiency) is used, the amount of oxygen included in theinsulating film 204 may be equal to that in the stoichiometriccomposition. However, in order to ensure reliability, for example, tosuppress variation in the threshold voltage of the transistor, theinsulating film 204 preferably includes oxygen at an amount which islarger than that in the stoichiometric composition, in consideration ofoxygen deficiency that may occur in the oxide semiconductor film.

In this manner, the oxygen doping treatment makes it easy to obtain agallium oxide film having a composition of Ga₂O_(3+α) (α>0), which canbe used as the insulating film 204, and to set a to be greater than orequal to 3.04 and less than or equal to 3.09. Alternatively, the oxygendoping treatment makes it easy to obtain an aluminum gallium oxide filmhaving a composition of Al_(x)Ga_(2-x)O_(3+α) (1<x<2, α>0), which can beused as the insulating film 204. Further alternatively, the oxygendoping treatment makes it easy to obtain a gallium aluminum oxide filmhaving a composition of Al_(x)Ga_(2-x)O_(3+α) (0<x≦1, α>0), which can beused as the insulating film 204.

Next, an oxide semiconductor film is formed over the insulating film 204and then processed, so that the oxide semiconductor film 206 having anisland shape is formed (see FIG. 1B).

As a material used for the oxide semiconductor film 206, afour-component metal oxide such as an In—Sn—Ga—Zn—O-based material; athree-component metal oxide such as an In—Ga—Zn—O-based material, anIn—Sn—Zn—O-based material, an In—Al—Zn—O-based material, aSn—Ga—Zn—O-based material, an Al—Ga—Zn—O-based material, or aSn—Al—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; a single-componentmetal oxide such as an In—O-based material, a Sn—O-based material, or aZn—O-based material, or the like can be used. In addition, the abovematerials may include SiO₂. Here, for example, an In—Ga—Zn—O-basedmaterial means an oxide film including indium (In), gallium (Ga), andzinc (Zn), and there is no particular limitation on the compositionratio thereof. Further, the In—Ga—Zn—O-based material may include anelement other than In, Ga, and Zn.

The oxide semiconductor film 206 can be a thin film formed using amaterial expressed by the chemical formula, InMO₃(ZnO)_(m) (m>0). Here,M represents one or more metal elements selected from Ga, Al, Mn, andCo. For example, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or thelike.

The thickness of the oxide semiconductor film 206 is preferably greaterthan or equal to 3 nm and less than or equal to 30 nm. This is becausethe transistor might be normally on when the oxide semiconductor film206 is too thick (e.g., the thickness is 50 nm or more).

It is preferable to form the oxide semiconductor film by a method withwhich impurities such as hydrogen, water, hydroxyl group, or hydride donot easily enter the oxide semiconductor film. For example, a sputteringmethod or the like can be used.

In this embodiment, the oxide semiconductor film is formed by asputtering method using an In—Ga—Zn—O-based oxide target.

As the In—Ga—Zn—O-based oxide target, for example, an oxide targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can beused. Note that it is not necessary to limit the material and thecomposition ratio of the target to the above. For example, an oxidetarget having a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio]can be used.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, a target to be used has a composition ratio of In:Zn=50:1to 1:2 in atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in atomic ratio (In₂O₃:ZnO=10:1 to 1:2 inmolar ratio), further preferably In:Zn=15:1 to 1.5:1 in atomic ratio(In₂O₃:ZnO=15:2 to 3:4 in molar ratio). For example, in a target usedfor formation of an In—Zn—O-based oxide semiconductor which has anatomic ratio of In:Zn:O=X:Y:Z, the relation of Z>1.5X+Y is satisfied.

The filling rate of the oxide target is higher than or equal to 90% andlower than or equal to 100%, preferably higher than or equal to 95% andlower than or equal to 99.9%. With the use of the metal oxide targetwith a high filling rate, a dense oxide semiconductor film can beformed.

The atmosphere for film formation may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, a mixed atmosphere of a rare gas andoxygen, or the like. Further, in order to prevent impurities such ashydrogen or water from entering the oxide semiconductor film, it ispreferable to use an atmosphere of a high-purity gas from whichimpurities such as hydrogen or water are sufficiently removed.

For example, the oxide semiconductor film can be formed as follows.

First, the substrate 200 is held in a deposition chamber kept underreduced pressure, and heating is performed so that the substratetemperature becomes higher than 200° C. and lower than or equal to 500°C., preferably higher than 300° C. and lower than or equal to 500° C.,further preferably higher than or equal to 350° C. and lower than orequal to 450° C.

Next, a high-purity gas from which impurities such as hydrogen or waterare sufficiently removed is introduced into the deposition chamber whilemoisture remaining therein is being removed, and the oxide semiconductorfilm is formed over the insulating film 204 with the use of the target.In order to remove moisture remaining in the deposition chamber, anentrapment vacuum pump such as a cryopump, an ion pump, or a titaniumsublimation pump is preferably used as an evacuation unit. Theevacuation unit may be a turbo pump provided with a cold trap. In thedeposition chamber which is evacuated with the cryopump, for example,impurities such as hydrogen or water (preferably, also a compoundincluding a carbon atom), and the like are removed, so that theconcentration of an impurity such as hydrogen or water included in theoxide semiconductor film formed in the deposition chamber can bereduced.

When the substrate temperature during film formation is low (e.g., lowerthan or equal to 100° C.), impurities such as hydrogen or water mightenter the oxide semiconductor film; therefore, the substrate 200 ispreferably heated as specified above. The substrate 200 is heated asspecified above and the oxide semiconductor film is formed. When theoxide semiconductor film is formed with the substrate 200 heated asspecified above, the substrate temperature is high, so that hydrogenbonds are cut by heat and hydrogen is less likely to be introduced intothe oxide semiconductor film. Therefore, by forming the oxidesemiconductor film with the substrate 200 heated as specified above, theconcentration of impurities such as hydrogen or water in the oxidesemiconductor film can be sufficiently reduced. Moreover, damage due tosputtering can be reduced.

Note that, as a method for measuring the amount of water in the oxidesemiconductor film, thermal desorption spectroscopy (TDS) is given. Forexample, when the temperature is increased from room temperature toapproximately 400° C., elimination of water, hydrogen, hydroxyl group,and the like in the oxide semiconductor film can be observed in therange of 200° C. to 300° C. approximately.

As an example of film formation conditions, the following conditions areemployed: the distance between the substrate and the target is 60 mm;the pressure is 0.4 Pa; the direct-current (DC) power is 0.5 kW; thesubstrate temperature is 400° C.; and the deposition atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulsed direct-current power source is preferably used becausepowder substances (also referred to as particles or dust) generated inthe film formation can be reduced and the film thickness can be uniform.

Note that before the oxide semiconductor film is formed by a sputteringmethod, powder substances (also referred to as particles or dust) whichare attached to a surface of the insulating film 204 are preferablyremoved by reverse sputtering in which an argon gas is introduced andplasma is generated. The reverse sputtering refers to a method in whichvoltage is applied to a substrate to generate plasma in the vicinity ofthe substrate so that a surface on the substrate side is modified. Notethat instead of argon, a gas of nitrogen, helium, oxygen, or the likemay be used.

The oxide semiconductor film can be processed by being etched after amask having a desired shape is formed over the oxide semiconductor film.The above mask can be formed by a method such as photolithography.Alternatively, a method such as an inkjet method may be used to form themask. For the etching of the oxide semiconductor film, either dryetching or wet etching may be employed. Needless to say, both of themmay be employed in combination.

The oxide semiconductor film 206 formed in such a manner may besubjected to heat treatment (first heat treatment). The heat treatmentfurther removes impurities such as hydrogen or water included in theoxide semiconductor film 206; thus, the structure of the oxidesemiconductor film 206 can be improved and defect levels in the energygap can be reduced.

The heat treatment is performed in an inert gas atmosphere at higherthan or equal to 250° C. and lower than or equal to 700° C., preferablyhigher than or equal to 450° C. and lower than or equal to 600° C. orlower than a strain point of the substrate. The inert gas atmosphere ispreferably an atmosphere which includes nitrogen or a rare gas (such ashelium, neon, or argon) as a main component and does not includeimpurities such as hydrogen or water. For example, the purity ofnitrogen or a rare gas such as helium, neon, or argon introduced into aheat treatment apparatus can be higher than or equal to 6N (99.9999%),preferably higher than or equal to 7N (99.99999%) (i.e., the impurityconcentration is lower than or equal to 1 ppm, preferably lower than orequal to 0.1 ppm).

The heat treatment can be performed in such a manner that, for example,an object to be heated is introduced into an electric furnace in which aresistance heating element or the like is used and heated at 450° C. forone hour in a nitrogen atmosphere. The oxide semiconductor film 206 isnot exposed to the air during the heat treatment so that entry ofimpurities such as hydrogen or water can be prevented.

The above heat treatment can also be referred to as dehydrationtreatment, dehydrogenation treatment, or the like because of its effectof removing impurities such as hydrogen or water. The heat treatment canbe performed at the timing, for example, after the oxide semiconductorfilm is formed. Such dehydration treatment or dehydrogenation treatmentmay be conducted once or plural times.

Next, treatment for supplying oxygen (also referred to as oxygen dopingtreatment, or the like) is preferably performed on the oxidesemiconductor film 206. As the treatment for supplying oxygen, heattreatment in an oxygen atmosphere (second heat treatment), treatmentusing oxygen plasma, and the like are given. Alternatively, oxygen maybe added by performing irradiation with an oxygen ion accelerated by anelectric field.

Note that an electric bias may be applied to the substrate in order toadd oxygen more favorably.

By performing the oxygen doping treatment on the oxide semiconductorfilm 206, oxygen can be included in the oxide semiconductor film 206or/and in the vicinity of the interface of the oxide semiconductor film206. In that case, the amount of oxygen is preferably larger than thatin the stoichiometric composition ratio of the oxide semiconductor film.

Note that, heat treatment may be performed on the oxide semiconductorfilm 206 which has been subjected to the oxygen doping treatment. Theheat treatment is performed at higher than or equal to 250° C. and lowerthan or equal to 700° C., preferably higher than or equal to 400° C. andlower than or equal to 600° C. or lower than the strain point of thesubstrate.

Through the heat treatment, water, hydroxide (OH), and the likegenerated by reaction between oxygen and an oxide semiconductor materialcan be removed from the oxide semiconductor film. By this heattreatment, hydrogen or the like that has entered the oxide semiconductorfilm 206 or the like during the above oxygen doping treatment can alsobe removed. The heat treatment may be performed in an atmosphere ofnitrogen, oxygen, ultra-dry air (air where the moisture content is 20ppm (−55° C. by conversion into a dew point) or less, preferably 1 ppmor less, further preferably 10 ppb or less when measurement is performedusing a dew-point meter of a cavity ring down laser spectroscopy (CRDS)system), a rare gas (such as argon or helium), or the like in whichwater, hydrogen, and the like are sufficiently reduced. In particular,the heat treatment is preferably performed in an atmosphere includingoxygen. Further, the purity of nitrogen, oxygen, or a rare gasintroduced into a heat treatment apparatus is preferably higher than orequal to 6N (99.9999%) (i.e., the impurity concentration is lower thanor equal to 1 ppm), further preferably higher than or equal to 7N(99.99999%) (i.e., the impurity concentration is lower than or equal to0.1 ppm).

Note that the timing of the oxygen doping treatment is not limited tothe timing described above. However, the oxygen doping treatment ispreferably performed after the heat treatment for dehydration or thelike.

As described above, the heat treatment for dehydration or the like, andthe oxygen doping treatment or the heat treatment for supply of oxygenare performed, whereby the oxide semiconductor film 206 can be highlypurified so as to include elements (impurity elements) that are not maincomponents of the oxide semiconductor film 206 as few as possible. Thehighly purified oxide semiconductor film 206 includes extremely fewcarriers derived from a donor.

Next, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the insulating film204 and the oxide semiconductor film 206 and then processed, so that thesource electrode 208 a and the drain electrode 208 b are formed (seeFIG. 1B). Note that a channel length L of the transistor is determinedby the distance between the edges of the source electrode 208 a and thedrain electrode 208 b which are formed here.

As the conductive film used for the source electrode 208 a and the drainelectrode 208 b, for example, a metal film including an element selectedfrom aluminum, chromium, copper, tantalum, titanium, molybdenum, andtungsten; a metal nitride film including any of the above elements as acomponent (a titanium nitride film, a molybdenum nitride film, or atungsten nitride film); or the like can be used. The source electrode208 a and the drain electrode 208 b may have a single-layer structure ora stacked-layer structure. A high-melting-point metal film of titanium,molybdenum, tungsten, or the like or a metal nitride film thereof (atitanium nitride film, a molybdenum nitride film, or a tungsten nitridefilm) may be stacked on one or both of top and bottom sides of a metalfilm of aluminum, copper, or the like.

Alternatively, the conductive film used for the source electrode 208 aand the drain electrode 208 b may be formed using a conductive metaloxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide(SnO₂), zinc oxide (ZnO), indium oxide-tin oxide alloy (In₂O₃—SnO₂;abbreviated to ITO), indium oxide-zinc oxide alloy (In₂O₃—ZnO), or anyof these metal oxide materials including silicon oxide can be used.

The conductive film can be processed by being etched after a mask havinga desired shape is formed over the conductive film. As the above mask, aresist mask or the like can be used. Ultraviolet light, KrF laser light,ArF laser light, or the like is preferably used for light exposure atthe time of forming the resist mask.

In the case where the channel length L is less than 25 nm, the lightexposure at the time of forming the resist mask is preferably performedusing, for example, extreme ultraviolet light having an extremely shortwavelength of several nanometers to several tens of nanometers. In thelight exposure by extreme ultraviolet light, the resolution is high andthe focus depth is large. Thus, the channel length L of the transistorformed later can be reduced, whereby the operation speed of a circuitcan be increased.

An etching step may be performed using a resist mask formed using aso-called multi-tone mask. A resist mask formed using a multi-tone maskhas a plurality of thicknesses and can be further changed in shape byashing; thus, such a resist mask can be used in a plurality of etchingsteps for different patterns. Therefore, a resist mask corresponding toat least two kinds of different patterns can be formed by using onemulti-tone mask. In other words, the manufacturing process can besimplified.

Note that in the etching of the conductive film, part of the oxidesemiconductor film 206 is etched, so that the oxide semiconductor film206 has a groove (a recessed portion) in some cases.

After that, by plasma treatment using a gas such as N₂O, N₂, or Ar,impurities such as hydrogen or water attached to a surface of an exposedportion of the oxide semiconductor film 206 may be removed.

Next, the insulating film 210 is formed so as to cover the sourceelectrode 208 a and the drain electrode 208 b and be partly in contactwith the oxide semiconductor film 206, and the insulating film 212 isformed over and in contact with the insulating film 210. Then, a resistmask 214 is formed over the insulating film 212 (see FIG. 1C).

The insulating film 210 functions as a protective film of the transistor110. The insulating film 210 can be formed using a material includinggallium oxide. The material including gallium oxide may further includealuminum oxide; that is, a material including aluminum gallium oxide orgallium aluminum oxide, or the like may be used. In addition, thethickness of the insulating film 210 is preferably greater than or equalto 10 nm and less than 100 nm.

Here, a gallium oxide film used as the insulating film 210 can have acomposition of Ga₂O_(3+α) (α>0). It is preferable that a be greater thanor equal to 3.04 and less than or equal to 3.09. Alternatively, analuminum gallium oxide film used as the insulating film 210 preferablyhas a composition of Al_(x)Ga_(2-x)O_(3+α) (1<x<2, α>0). Furtheralternatively, a gallium aluminum oxide film used as the insulating film210 preferably has a composition of Al_(x)Ga_(2-x)O_(3+α) (0<x≦1, α>0)by being doped with oxygen.

The insulating film 210 is preferably formed by a method with whichimpurities such as hydrogen or water do not enter the insulating film210. This is because, when impurities such as hydrogen or water areincluded in the insulating film 210, the impurities such as hydrogen orwater enter the oxide semiconductor film formed later or oxygen in theoxide semiconductor film is extracted by the impurities such as hydrogenor water, so that a back channel of the oxide semiconductor film mighthave lower resistance (have n-type conductivity) and a parasitic channelmight be formed. Therefore, the insulating film 210 is preferably formedso as to include impurities such as hydrogen or water as few aspossible. For example, the insulating film 210 is preferably formed by asputtering method. A high-purity gas from which impurities such ashydrogen or water are removed is preferably used as a sputtering gasused for film formation.

Examples of the sputtering method include a DC sputtering method inwhich a direct current power source is used, a pulsed DC sputteringmethod in which a direct bias is applied in a pulsed manner, an ACsputtering method, and the like.

Note that the insulating film 210 is preferably formed while thesubstrate 200 is being heated.

In many cases, an oxide semiconductor material used for the oxidesemiconductor film includes gallium. Therefore, the insulating film 210which is in contact with the oxide semiconductor film is formed using amaterial including gallium oxide, whereby a favorable condition can bemaintained at an interface between the oxide semiconductor film and theinsulating film 210. For example, by providing the oxide semiconductorfilm in contact with the insulating film 210 including gallium oxide, anaccumulation of hydrogen at an interface between the oxide semiconductorfilm and the insulating film can be reduced. This is because a materialincluding gallium oxide is compatible with an oxide semiconductormaterial.

Note that, in the case where an element belonging to the same group as aconstituent element of the oxide semiconductor is used for theinsulating film 210, a similar effect can be obtained. That is, it isalso effective to additionally use a material including aluminum oxideor the like for the insulating film 210. Aluminum oxide tends to preventwater penetration; therefore, it is preferable to use the materialincluding aluminum oxide in the optic of preventing water from enteringthe oxide semiconductor film. For example, a material including galliumand aluminum, such as aluminum gallium oxide (or gallium aluminum oxide)may be used for the insulating film 210. In this case, both the effectresulting from inclusion of gallium and the effect resulting frominclusion of aluminum can be obtained, which is preferable. By providingthe oxide semiconductor film in contact with an insulating filmincluding aluminum gallium oxide, for example, water can be preventedfrom entering the oxide semiconductor film and an accumulation ofhydrogen at an interface between the oxide semiconductor film and theinsulating film can be sufficiently reduced.

Note that oxygen doping treatment may be performed on the insulatingfilm 210 here. Oxygen doping refers to addition of oxygen (whichincludes at least one of an oxygen radical, an oxygen atom, and anoxygen ion) to a bulk. Note that the term “bulk” is used in order toclarify that oxygen is added not only to a surface of a thin film butalso to the inside of the thin film. In addition, the oxygen dopingincludes oxygen plasma doping in which oxygen that is made to be plasmais added to a bulk.

When the oxygen doping treatment is performed on the insulating film210, a region where the amount of oxygen is larger than that in thestoichiometric composition ratio is formed in the insulating film 210.By providing such a region, oxygen can be supplied to the oxidesemiconductor film and oxygen deficiency defects in the oxidesemiconductor film can be reduced.

Note that in the case where an oxide semiconductor without defects(oxygen deficiency) is used, the amount of oxygen included in theinsulating film 210 may be equal to that in the stoichiometriccomposition. However, in order to ensure reliability, for example, tosuppress variation in the threshold voltage of the transistor, theinsulating film 210 preferably includes oxygen at an amount which islarger than that in the stoichiometric composition, in consideration ofoxygen deficiency that may occur in the oxide semiconductor film.

In this manner, the oxygen doping treatment makes it easy to obtain agallium oxide film having a composition of Ga₂O_(3+α) (α>0), which canbe used as the insulating film 210, and to set a to be greater than orequal to 3.04 and less than or equal to 3.09. Alternatively, the oxygendoping treatment makes it easy to obtain an aluminum gallium oxide filmhaving a composition of Al_(x)Ga_(2-x)O_(3+α) (1<x<2, α>0), which can beused as the insulating film 210. Further alternatively, the oxygendoping treatment makes it easy to obtain a gallium aluminum oxide filmhaving a composition of Al_(x)Ga_(2-x)O_(3+α) (0<x≦1, α>0), which can beused as the insulating film 210.

The insulating film 212 also functions as a protective film of thetransistor 110. The insulating film 212 can be formed by a plasma CVDmethod using a material such as silicon oxide, silicon nitride, siliconoxynitride, or silicon nitride oxide or a sputtering method, forexample. In addition, the thickness of the insulating film 212 is largerthan the thickness of the insulating film 210 and is preferably greaterthan or equal to 100 nm and less than or equal to 300 nm.

The insulating film 210 and the insulating film 212 having a largerthickness than the insulating film 210 are stacked in this manner, sothat a favorable condition can be maintained at the interface betweenthe oxide semiconductor film and the protective film by the insulatingfilm 210 including gallium oxide and an adequate thickness for theprotective film of the transistor 110 can be secured by the insulatingfilm 212. Further, by thus increasing the thickness of the stack of theinsulating film 210 and the insulating film 212, which functions as theprotective film of the transistor 110, parasitic capacitance between thetransistor and a wiring formed in a subsequent step can be reduced.

The resist mask 214 can be formed using a material such as aphotosensitive resin by a photolithography method. Here, if theinsulating film 210 including gallium oxide is subjected to wettreatment using a developer or the like, the region which is in theinsulating film 210 and is needed in design might be dissolved. However,by stacking the insulating film 210 and the insulating film 212 asdescribed in this embodiment and providing the resist mask 214 over theinsulating film 212, the resist mask 214 can be formed without theinsulating film 210 including gallium oxide being in contact with thedeveloper. Thus, the resist mask 214 can be formed without a possibilitythat the region which is in the insulating film 210 including galliumoxide and is needed in design is dissolved.

Next, dry etching is performed on the insulating film 210 and theinsulating film 212 with the use of the resist mask 214, so that thecontact hole 218 is formed (see FIG. 1D).

If wet treatment such as wet etching is performed here, the region whichis in the insulating film 210 including gallium oxide and is needed indesign might be dissolved. Therefore, particularly in the case ofminiaturizing the semiconductor device, it is extremely difficult toprocess the insulating film 210 including gallium oxide by wet etchingto have a designed size.

Thus, the insulating film 210 and the insulating film 212 are processedby dry etching in this embodiment. For the dry etching, a parallel platereactive ion etching (RIE) method, an inductively coupled plasma (ICP)etching method, or the like can be used. Etching conditions (such as theamount of power applied to a coil-shaped electrode, the amount of powerapplied to an electrode on a substrate side, and the temperature of theelectrode on the substrate side) may be set as appropriate in accordancewith the material so that the films can be etched into desired shapes.

As an etching gas that can be used for the dry etching, a gas includingfluorine (a fluorine-based gas such as carbon tetrafluoride (CF₄),sulfur hexafluoride (SF₆), nitrogen trifluoride (NF₃), ortrifluoromethane (CHF₃)); octafluorocyclobutane (C₄F₈); any of thesegases to which a rare gas such as helium (He) or argon (Ar) is added;and the like can be given. By etching the insulating film 210 and theinsulating film 212 with the use of such a gas including fluorine,etching can be performed such that etching selectivity of the insulatingfilms with respect to the source electrode 208 a is high. For example,conditions of etching using an ICP etching method are set as follows:7.5 sccm of trifluoromethane and 142.5 sccm of helium are used as anetching gas; the power applied to the coil-shaped electrode is 475 W;the power applied to the electrode on the substrate side is 300 W; thepressure is 5.5 Pa; and the temperature of a lower electrode is 70° C.

Further, a gas including chlorine (a chlorine-based gas such as chlorine(Cl₂), boron trichloride (BCl₃), silicon tetrachloride (SiCl₄), orcarbon tetrachloride (CCl₄)) or the like may be used. In the case wherean insulating film including gallium oxide and aluminum oxide is used asthe insulating film 210, the insulating film 210 can be easily etchedusing such a gas including chlorine.

In the dry etching, the etching rate of the insulating film 210including gallium oxide tends to be lower than the etching rate of theinsulating film 212. Accordingly, the diameter of the contact hole 218in the insulating film 210 can be easily made smaller than the diameterof the contact hole 218 in the insulating film 212 as illustrated inFIG. 1D. That is, the contact hole 218 can have a stepped shape. Byforming the contact hole 218 having such a stepped shape, coverage withthe wiring 216 formed later can be improved. Alternatively, the contacthole 218 may be formed to have a shape where the diameter of the contacthole 218 in the insulating film 210 is substantially equal to thediameter of the contact hole 218 in the insulating film 212 by settingetching conditions (such as an etching gas, the etching time, and thetemperature) as appropriate so that the insulating film 210 and theinsulating film 212 have the same or substantially the same etchingrate.

By performing dry etching on the insulating film 210 and the insulatingfilm 212 in this manner, the contact hole 218 can be formed in theinsulating film 210 and the insulating film 212 without a possibilitythat the region which is in the insulating film 210 including galliumoxide and is needed in design is dissolved.

Next, the resist mask 214 is removed by ashing using oxygen plasma, andthe wiring 216 is formed so as to be electrically connected to thesource electrode 208 a through the contact hole 218 (see FIG. 1E).

Here, if an alkali stripper solution is used for the removal of theresist mask 214, the region which is in the insulating film 210including gallium oxide and is needed in design might be dissolved.Therefore, in this embodiment, the resist mask 214 is removed by ashingusing oxygen plasma.

The ashing treatment using oxygen plasma is performed in an oxygenatmosphere in such a manner that oxygen is made to be plasma by highfrequency power or the like and the resist mask 214 is decomposed andremoved by the oxygen that is made to be plasma. By removing the resistmask 214 in this manner, the resist mask 214 can be removed without apossibility that the region which is in the insulating film 210including gallium oxide and is needed in design is dissolved. Forexample, conditions of the ashing treatment using oxygen plasma, inwhich an ICP apparatus is used, are set as follows: 300 sccm of anoxygen gas is used; the power of an RF power source is 1800 W; thepressure is 66.5 Pa; and the treatment duration is 180 seconds.

By the ashing treatment using oxygen plasma, the resist mask 214 can beremoved without generation of a residue of the resist mask 214 or areaction product of the residue. Accordingly, a surface of theinsulating film 212 can be kept clean after the resist mask 214 isremoved.

The wiring 216 is electrically connected to the source electrode 208 aof the transistor 110 and functions as a so-called source wiring. Thewiring 216 can be formed using a material and a method similar to thoseof the source electrode 208 a and the drain electrode 208 b.

As described above, even when an insulating film formed using a materialincluding gallium oxide is used, the insulating film is not directlysubjected to wet treatment; thus, the transistor 110 can be formedwithout a possibility that a region which is in the insulating filmincluding gallium oxide and is needed in design is dissolved (see FIG.1E).

Note that in the manufacturing process of the transistor 110 in FIGS. 1Ato 1E, the step of electrically connecting the wiring 216 to the sourceelectrode 208 a is described; however, the disclosed invention is notlimited to this. For example, a wiring formed over the insulating film212 may be electrically connected to the drain electrode 208 b by asimilar method. In the case where the transistor is used for a pixelportion, a pixel electrode may be electrically connected to the sourceelectrode 208 a or the drain electrode 208 b by a similar method. Inaddition, a wiring formed over the insulating film 212 may beelectrically connected to the gate electrode (or a wiring formed in thesame layer as the gate electrode) by a similar method.

Here, a step in which a wiring 222 that is electrically connected to thegate electrode 202 (or a wiring formed in the same layer as the gateelectrode 202) illustrated in FIGS. 1A to 1E through a contact hole 224is formed over the insulating film 212 will be described with referenceto FIGS. 2A to 2C. Here, the gate electrode 202 and the wiring 222 areelectrically connected to each other in a region where the gateelectrode 202 does not overlap with the oxide semiconductor film 206,the source electrode 208 a, or the drain electrode 208 b.

First, steps similar to those comprised in the manufacturing process ofthe transistor 110 are performed up to and including the stepillustrated in FIG. 1C; thus, the resist mask 214 is formed so that thecontact hole 224 can be formed in a region where the gate electrode 202does not overlap with the oxide semiconductor film 206, the sourceelectrode 208 a, and the drain electrode 208 b (see FIG. 2A).

Next, dry etching is performed on the insulating film 204, theinsulating film 210, and the insulating film 212 by a method similar tothat used in the step illustrated in FIG. 1D, so that the contact hole224 is formed (see FIG. 2B). Here, the insulating film 204 is alsoetched in the same step, which is different from the step illustrated inFIG. 1D; when the insulating film 204 is formed using a material similarto that of the insulating film 210 or the insulating film 212, theinsulating film 204 can be etched in a manner similar to that of theinsulating film 210 or the insulating film 212.

Then, the resist mask 214 is removed by ashing using oxygen plasma by amethod similar to that used in the step illustrated in FIG. 1E, and thewiring 222 is formed so as to be electrically connected to the gateelectrode 202 through the contact hole 224 (see FIG. 2C). Here, thewiring 222 functions as a so-called gate wiring and can be formed usinga method and a material similar to those of the wiring 216 illustratedin FIG. 1E.

As described above, even when an insulating film formed using a materialincluding gallium oxide is used, the insulating film is not directlysubjected to wet treatment; thus, the wiring 222 which is electricallyconnected to the gate electrode 202 (including the wiring formed in thesame layer as the gate electrode 202) through the contact hole 224 canbe formed over the insulating film 212 without a possibility that aregion which is in the insulating film including gallium oxide and isneeded in design is dissolved.

In addition, as the manufacturing process of the transistor 110 in FIGS.1A to 1E, a process for manufacturing the bottom-gate transistor 110 inwhich the gate electrode 202 is provided below the oxide semiconductorfilm 206 to overlap therewith is described; however, the disclosedinvention is not limited to this. For example, a top-gate transistor inwhich a gate electrode is provided over an oxide semiconductor film tooverlap therewith may be manufactured by a similar method.

Here, an example of a manufacturing process of a top-gate transistor 310illustrated in FIG. 3E will be described with reference to FIGS. 3A to3E. Here, the transistor 310 includes, over a substrate 400, an oxidesemiconductor film 406, a source electrode 408 a, a drain electrode 408b, an insulating film 410, a gate electrode 402, an insulating film 412,and a wiring 416. In the transistor illustrated in FIG. 3E, theinsulating film 410 including gallium oxide is provided in contact withthe oxide semiconductor film 406, and the insulating film 412 isprovided over and in contact with the insulating film 410 includinggallium oxide. In addition, the wiring 416 is electrically connected tothe source electrode 408 a through a contact hole 418 formed in theinsulating film 412 and the insulating film 410.

First, an oxide semiconductor film for forming the oxide semiconductorfilm 406 is formed over the substrate 400 and then processed into anisland shape, so that the oxide semiconductor film 406 is formed. Afterthat, a conductive film for forming the source electrode and the drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over and in contact withthe oxide semiconductor film 406. The conductive film is processed, sothat the source electrode 408 a and the drain electrode 408 b are formed(see FIG. 3A). The substrate 400, the oxide semiconductor film 406, thesource electrode 408 a, and the drain electrode 408 b can be formedusing materials and methods similar to those of the substrate 200, theoxide semiconductor film 206, the source electrode 208 a, and the drainelectrode 208 b illustrated in FIGS. 1A to 1E.

Note that a base insulating film is preferably formed over the substrate400 before the oxide semiconductor film 406 is formed. The baseinsulating film can be formed using a material and a method similar tothose of the insulating film 204 illustrated in FIGS. 1A to 1E, and ispreferably formed using a material including gallium oxide or a materialincluding gallium oxide and aluminum oxide. Thus, a favorable conditioncan be maintained at an interface where the base insulating film is incontact with the oxide semiconductor film.

Next, the insulating film 410 is formed so as to cover the oxidesemiconductor film 406, the source electrode 408 a, and the drainelectrode 408 b. Then, a conductive film for forming the gate electrode(including a wiring formed in the same layer as the gate electrode) isformed and then processed, so that the gate electrode 402 is formed tooverlap with the oxide semiconductor film 406 (see FIG. 3B). The gateelectrode 402 can be formed using a material and a method similar tothose of the gate electrode 202 illustrated in FIGS. 1A to 1E. Theinsulating film 410 can be formed using a material and a method similarto those of the insulating film 210 illustrated in FIGS. 1C to 1E.Further, the insulating film 410 may have a stacked-layer structureincluding an insulating film formed using a material similar to that ofthe insulating film 204 illustrated in FIGS. 1A to 1E. In this manner,by using a material including gallium oxide for the insulating film 410,a favorable condition can be maintained at an interface where theinsulating film 410 is in contact with the oxide semiconductor film.

Next, the insulating film 412 is formed so as to cover the gateelectrode 402 and insulating film 410, and then a resist mask 414 isformed over the insulating film 412 (see FIG. 3C). The insulating film412 and the resist mask 414 can be formed using materials and methodssimilar to those of the insulating film 212 and the resist mask 214illustrated in FIGS. 1C to 1E.

Next, dry etching is performed on the insulating film 410 and theinsulating film 412 with the use of the resist mask 414, so that thecontact hole 418 is formed (see FIG. 3D). The contact hole 418 can beformed by a method similar to that of the contact hole 218 illustratedin FIGS. 1D and 1E.

Then, the resist mask 414 is removed by ashing using oxygen plasma, andthe wiring 416 is formed so as to be electrically connected to thesource electrode 408 a through the contact hole 418 (see FIG. 3E). Theresist mask 414 can be removed by a method similar to that used in theremoval of the resist mask 214 illustrated in the FIGS. 1C and 1D. Thewiring 416 can be formed using a material and a method similar to thoseof the wiring 216 illustrated in FIG. 1E.

As described above, even when an insulating film formed using a materialincluding gallium oxide is used, the insulating film is not directlysubjected to wet treatment; thus, the transistor 310 can be formedwithout possibility that a region in the insulating film includinggallium oxide and needed in design is dissolved (see FIG. 3E).

As the manufacturing process of the transistor illustrated in FIGS. 1Ato 1E or FIGS. 3A to 3E, a process for manufacturing the transistor inwhich at least a top surface of the oxide semiconductor film iselectrically connected to the source electrode or the drain electrode isdescribed; however, the disclosed invention is not limited to this. Forexample, a transistor in which at least a bottom surface of an oxidesemiconductor film is electrically connected to a source electrode or adrain electrode may be manufactured by a similar method.

In this embodiment, etching treatment is performed on the insulatingfilms obtained by providing, over and in contact with the insulatingfilm including gallium oxide, another insulating film; however, thedisclosed invention is not limited to this.

As described in this embodiment, an insulating film which is in contactwith an oxide semiconductor film is formed using a material includinggallium oxide, whereby a favorable condition can be maintained at aninterface between the oxide semiconductor film and the insulating film.

In an etching step of the insulating film formed using a materialincluding gallium oxide, the insulating film is not directly subjectedto wet treatment; thus, the insulating film including gallium oxide canbe etched without possibility that a region in the insulating filmincluding gallium oxide and needed in design is dissolved.

Accordingly, a semiconductor device including an oxide semiconductor andhaving stable electric characteristics can be provided. Therefore, asemiconductor device having high reliability can be provided.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 2

A semiconductor device having a display function (also referred to as adisplay device) can be manufactured using the transistor described inEmbodiment 1. Moreover, part or all of a driver circuit which includesthe transistor can be formed over a substrate where a pixel portion isformed, whereby a system-on-panel can be obtained.

In FIG. 5A, a sealant 4005 is provided so as to surround a pixel portion4002 provided over a first substrate 4001, and the pixel portion 4002 issealed by using a second substrate 4006. In FIG. 5A, a signal linedriver circuit 4003 and a scan line driver circuit 4004 which are formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared are mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. Various signals and potentials aresupplied from flexible printed circuits (FPCs) 4018 a and 4018 b to thesignal line driver circuit 4003 and the scan line driver circuit 4004,which are separately formed, and the pixel portion 4002.

In FIGS. 5B and 5C, the sealant 4005 is provided so as to surround thepixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001. The second substrate 4006 isprovided over the pixel portion 4002 and the scan line driver circuit4004. Consequently, the pixel portion 4002 and the scan line drivercircuit 4004 are sealed together with a display element, by the firstsubstrate 4001, the sealant 4005, and the second substrate 4006. InFIGS. 5B and 5C, the signal line driver circuit 4003 which is formedusing a single crystal semiconductor film or a polycrystallinesemiconductor film over a substrate separately prepared is mounted in aregion that is different from the region surrounded by the sealant 4005over the first substrate 4001. In FIGS. 5B and 5C, various signals andpotentials are supplied from an FPC 4018 to the separately formed signalline driver circuit 4003, the scan line driver circuit 4004, and thepixel portion 4002.

Although FIGS. 5B and 5C each illustrate an example in which the signalline driver circuit 4003 is formed separately and mounted on the firstsubstrate 4001, the disclosed invention is not limited to thisstructure. The scan line driver circuit may be separately formed andthen mounted, or only part of the signal line driver circuit or part ofthe scan line driver circuit may be separately formed and then mounted.

Note that there is no particular limitation on the connection method ofa separately formed driver circuit, and a chip on glass (COG) method, awire bonding method, a tape automated bonding (TAB) method, or the likecan be used. FIG. 5A illustrates an example in which the signal linedriver circuit 4003 and the scan line driver circuit 4004 are mounted bya COG method. FIG. 5B illustrates an example in which the signal linedriver circuit 4003 is mounted by a COG method. FIG. 5C illustrates anexample in which the signal line driver circuit 4003 is mounted by a TABmethod.

In addition, the display device includes a panel in which the displayelement is sealed, and a module in which an IC including a controller orthe like is mounted on the panel.

Note that a display device in this specification means an image displaydevice, a display device, or a light source (including a lightingdevice). Further, the display device includes the following modules inits category: a module to which a connector such as an FPC, a TAB tape,or a TCP is attached; a module having a TAB tape or a TCP at the tip ofwhich a printed wiring board is provided; and a module in which anintegrated circuit (IC) is directly mounted on a display element by aCOG method.

Further, the pixel portion and the scan line driver circuit which areprovided over the first substrate include a plurality of transistors, towhich the transistor described in Embodiment 1 can be applied.

As the display element provided in the display device, a liquid crystalelement (also referred to as a liquid crystal display element) or alight-emitting element (also referred to as a light-emitting displayelement) can be used. The light-emitting element includes, in itscategory, an element whose luminance is controlled by current orvoltage, and specifically includes an inorganic electroluminescent (EL)element, an organic EL element, and the like. Furthermore, a displaymedium whose contrast is changed by an electric effect, such aselectronic ink, can be used.

One embodiment of a semiconductor device will be described withreference to FIG. 6, FIG. 7, and FIG. 8. FIG. 6, FIG. 7, and FIG. 8correspond to cross-sectional views along line M-N in FIG. 5B.

As illustrated in FIG. 6, FIG. 7, and FIG. 8, the semiconductor deviceincludes a connection terminal electrode 4015 and a terminal electrode4016. The connection terminal electrode 4015 and the terminal electrode4016 are electrically connected to a terminal included in the FPC 4018through an anisotropic conductive film 4019.

The connection terminal electrode 4015 is formed using the sameconductive film as a first electrode layer 4030. The terminal electrode4016 is formed using the same conductive film as source electrodes anddrain electrodes of a transistor 4010 and a transistor 4011.

The pixel portion 4002 and the scan line driver circuit 4004 which areprovided over the first substrate 4001 include a plurality oftransistors. In FIG. 6, FIG. 7, and FIG. 8, the transistor 4010 includedin the pixel portion 4002 and the transistor 4011 included in the scanline driver circuit 4004 are illustrated as an example.

In this embodiment, the transistor described in Embodiment 1 can beapplied to the transistor 4010 and the transistor 4011. Variation inelectric characteristics of the transistor 4010 and the transistor 4011is suppressed and the transistor 4010 and the transistor 4011 areelectrically stable. Accordingly, semiconductor devices having highreliability can be provided as the semiconductor devices of thisembodiment illustrated in FIG. 6, FIG. 7, and FIG. 8.

The transistor 4010 provided in the pixel portion 4002 is electricallyconnected to a display element to form a display panel. A variety ofdisplay elements can be used as the display element without particularlimitation as long as display can be performed.

An example of a liquid crystal display device using a liquid crystalelement as the display element is illustrated in FIG. 6. In FIG. 6, aliquid crystal element 4013 which is a display element includes thefirst electrode layer 4030, a second electrode layer 4031, and a liquidcrystal layer 4008. An insulating film 4032 and an insulating film 4033which function as alignment films are provided so that the liquidcrystal layer 4008 is sandwiched therebetween. The second electrodelayer 4031 is provided on the second substrate 4006 side, and the firstelectrode layer 4030 and the second electrode layer 4031 are stackedwith the liquid crystal layer 4008 positioned therebetween.

A columnar spacer denoted by reference numeral 4035 is obtained byselective etching of an insulating film and is provided in order tocontrol the thickness (a cell gap) of the liquid crystal layer 4008.Alternatively, a spherical spacer may be used.

In the case where a liquid crystal element is used as the displayelement, thermotropic liquid crystal, low-molecular liquid crystal,high-molecular liquid crystal, polymer dispersed liquid crystal,ferroelectric liquid crystal, anti-ferroelectric liquid crystal, or thelike can be used. Such a liquid crystal material exhibits a cholestericphase, a smectic phase, a cubic phase, a chiral nematic phase, anisotropic phase, or the like depending on requirements.

Alternatively, liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase is generated only in a narrowtemperature range, a liquid crystal composition in which 5 wt. % or moreof a chiral agent is mixed is used for the liquid crystal layer in orderto improve the temperature range. The liquid crystal composition whichincludes liquid crystal exhibiting a blue phase and a chiral agent has ashort response time of 1 millisecond or less, has optical isotropy,which makes an alignment process unneeded, and has a small viewing angledependence. In addition, since an alignment film does not need to beprovided and rubbing treatment is unnecessary, electrostatic dischargedamage caused by the rubbing treatment can be prevented and defects anddamage of the liquid crystal display device can be reduced in themanufacturing process. Thus, productivity of the liquid crystal displaydevice can be improved.

The specific resistivity of the liquid crystal material is 1×10⁹ Ω·cm orhigher, preferably 1×10¹¹ Ω·cm or higher, further preferably 1×10¹² Ω·cmor higher. The value of the specific resistivity in this specificationis measured at 20° C.

The size of a storage capacitor formed in the liquid crystal displaydevice is set considering the leakage current or the like of thetransistor provided in the pixel portion so that electric charge can beheld for a predetermined period. By using the transistor including thehigh-purity oxide semiconductor film, it is satisfactory to provide astorage capacitor having a capacitance that is ⅓ or less, preferably ⅕or less with respect to liquid crystal capacitance of each pixel.

In the transistor including the highly purified oxide semiconductor filmused in this embodiment, the current in an off state (the off-statecurrent) can be reduced. Therefore, an electrical signal such as animage signal can be held for a longer period, and a writing interval canbe set longer when the power is on. Accordingly, frequency of refreshoperation can be reduced, which leads to an effect of suppressing powerconsumption.

In addition, the transistor including the highly purified oxidesemiconductor film used in this embodiment can have relatively highfield-effect mobility and thus is capable of high speed operation.Therefore, by using the transistor in the pixel portion of the liquidcrystal display device, a high-quality image can be provided. Further,since the transistor can be separately provided in a driver circuitportion and a pixel portion over one substrate, the number of componentsof the liquid crystal display device can be reduced.

For the liquid crystal display device, a twisted nematic (TN) mode, anin-plane-switching (IPS) mode, a fringe field switching (FFS) mode, anaxially symmetric aligned micro-cell (ASM) mode, an optical compensatedbirefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, anantiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissiveliquid crystal display device utilizing a vertical alignment (VA) modemay be used. The vertical alignment mode is a method of controlling thealignment of liquid crystal molecules of a liquid crystal display panel,in which liquid crystal molecules are aligned vertically to a panelsurface when no voltage is applied. Some examples are given as thevertical alignment mode. For example, a multi-domain vertical alignment(MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, orthe like can be used. Moreover, it is possible to use a method calleddomain multiplication or multi-domain design, in which a pixel isdivided into some regions (subpixels) and molecules are aligned indifferent directions in their respective regions.

In the display device, a black matrix (a light-blocking layer), anoptical member (an optical substrate) such as a polarizing member, aretardation member, or an anti-reflection member, and the like areprovided as appropriate. For example, circular polarization may beemployed by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

In addition, it is possible to employ a time-division display method (afield-sequential driving method) with the use of a plurality oflight-emitting diodes (LEDs) as a backlight. By employing afield-sequential driving method, color display can be performed withoutusing a color filter.

As a display method in the pixel portion, a progressive method, aninterlace method, or the like can be employed. Further, color elementscontrolled in a pixel at the time of color display are not limited tothree colors: R, G, and B (R, G, and B correspond to red, green, andblue, respectively). For example, R, G, B, and W (W corresponds towhite); or R, G, B, and one or more of yellow, cyan, magenta, and thelike can be used. Further, the sizes of display regions may be differentbetween respective dots of color elements. The present invention is notlimited to the application to a display device for color display but canalso be applied to a display device for monochrome display.

Alternatively, as the display element included in the display device, alight-emitting element utilizing electroluminescence can be used.Light-emitting elements utilizing electroluminescence are classifiedaccording to whether a light-emitting material is an organic compound oran inorganic compound. In general, the former is referred to as anorganic EL element, and the latter is referred to as an inorganic ELelement.

In an organic EL element, by application of voltage to a light-emittingelement, electrons and holes are separately injected from a pair ofelectrodes into a layer including a light-emitting organic compound, andcurrent flows. The carriers (electrons and holes) are recombined, andthus the light-emitting organic compound is excited. The light-emittingorganic compound returns to a ground state from the excited state,thereby emitting light. Owing to such a mechanism, this light-emittingelement is referred to as a current-excitation light-emitting element.

Inorganic EL elements are classified according to their elementstructures into a dispersion-type inorganic EL element and a thin-filminorganic EL element. A dispersion-type inorganic EL element has alight-emitting layer where particles of a light-emitting material aredispersed in a binder, and its light emission mechanism isdonor-acceptor recombination type light emission that utilizes a donorlevel and an acceptor level. A thin-film inorganic EL element has astructure where a light-emitting layer is sandwiched between dielectriclayers, which are further sandwiched between electrodes, and its lightemission mechanism is localized type light emission that utilizesinner-shell electron transition of metal ions. Note that an example ofan organic EL element is described here as a light-emitting element.

In order to extract light emitted from the light-emitting element, atleast one of a pair of electrodes is transparent. Then, a transistor anda light-emitting element are formed over a substrate. The light-emittingelement can have any of the following structures: a top emissionstructure in which light is extracted through a surface opposite to thesubstrate; a bottom emission structure in which light is extractedthrough a surface on the substrate side; or a dual emission structure inwhich light is extracted through the surface opposite to the substrateand the surface on the substrate side.

An example of a light-emitting device in which a light-emitting elementis used as the display element is illustrated in FIG. 7. Alight-emitting element 4513 which is a display element is electricallyconnected to the transistor 4010 provided in the pixel portion 4002. Astructure of the light-emitting element 4513 is not limited to thestacked-layer structure including the first electrode layer 4030, anelectroluminescent layer 4511, and the second electrode layer 4031. Thestructure of the light-emitting element 4513 can be changed asappropriate depending on the direction in which light is extracted fromthe light-emitting element 4513, or the like.

A partition wall 4510 is formed using an organic insulating material oran inorganic insulating material. It is particularly preferable that thepartition wall 4510 be formed using a photosensitive resin material tohave an opening over the first electrode layer 4030 so that a sidewallof the opening is formed as a tilted surface with continuous curvature.

The electroluminescent layer 4511 may be formed using a single layer ora plurality of layers stacked.

A protective film may be formed over the second electrode layer 4031 andthe partition wall 4510 in order to prevent entry of oxygen, hydrogen,moisture, carbon dioxide, or the like into the light-emitting element4513. As the protective film, a silicon nitride film, a silicon nitrideoxide film, a DLC film, or the like can be formed. In a space which isformed with the first substrate 4001, the second substrate 4006, and thesealant 4005, a filler 4514 is provided for sealing. It is preferablethat a panel be packaged (sealed) with a protective film (such as alaminate film or an ultraviolet curable resin film) or a cover materialwith high air-tightness and little degasification so that the panel isnot exposed to the outside air, in this manner.

As the filler 4514, an ultraviolet curable resin or a thermosettingresin as well as an inert gas such as nitrogen or argon can be used. Forexample, polyvinyl chloride (PVC), acrylic, polyimide, an epoxy resin, asilicone resin, polyvinyl butyral (PVB), or ethylene vinyl acetate (EVA)can be used. For example, nitrogen may be used for the filler.

In addition, as needed, an optical film such as a polarizing plate, acircularly polarizing plate (including an elliptically polarizingplate), a retardation plate (a quarter-wave plate or a half-wave plate),or a color filter may be provided as appropriate on a light-emittingsurface of the light-emitting element. Further, the polarizing plate orthe circularly polarizing plate may be provided with an anti-reflectionfilm. For example, anti-glare treatment by which reflected light isdiffused by projections and depressions on the surface so that the glarecan be reduced can be performed.

Further, electronic paper in which electronic ink is driven can beprovided as the display device. The electronic paper is also referred toas an electrophoretic display device (an electrophoretic display) and isadvantageous in that it has the same level of readability as plainpaper, it has lower power consumption than other display devices, and itcan be made thin and lightweight.

An electrophoretic display device can have various modes. Anelectrophoretic display device includes a plurality of microcapsulesdispersed in a solvent or a solute; each microcapsule includes firstparticles which are positively charged and second particles which arenegatively charged. By applying an electric field to the microcapsules,the particles in the microcapsules move in opposite directions to eachother and only the color of the particles gathering on one side isdisplayed. Note that the first particles and the second particles eachinclude pigment and do not move without an electric field. Further, thefirst particles and the second particles have different colors (whichmay be colorless).

Thus, an electrophoretic display device is a display that utilizes aso-called dielectrophoretic effect by which a substance having a highdielectric constant moves to a high-electric field region.

A solution in which the above microcapsules are dispersed in a solventis referred to as electronic ink. This electronic ink can be printed ona surface of glass, plastic, cloth, paper, or the like. Furthermore, byusing a color filter or particles that have pigment, color display canbe achieved.

Note that the first particles and the second particles in themicrocapsules may each be formed using a single material selected from aconductive material, an insulating material, a semiconductor material, amagnetic material, a liquid crystal material, a ferroelectric material,an electroluminescent material, an electrochromic material, and amagnetophoretic material, or formed using a composite material of any ofthese.

As the electronic paper, a display device using a twisting ball displaysystem can be used. The twisting ball display system refers to a methodin which spherical particles each colored in black and white arearranged between a first electrode layer and a second electrode layerwhich are electrode layers used for a display element, and a potentialdifference is generated between the first electrode layer and the secondelectrode layer to control the orientation of the spherical particles,so that display is performed.

FIG. 8 illustrates active matrix electronic paper as one embodiment of asemiconductor device. The electronic paper in FIG. 8 is an example of adisplay device using a twisting ball display system.

Between the first electrode layer 4030 connected to the transistor 4010and the second electrode layer 4031 provided on the second substrate4006, spherical particles 4613 each of which includes a black region4615 a, a white region 4615 b, and a cavity 4612 around the regionswhich is filled with liquid are provided. A space around the sphericalparticles 4613 is filled with a filler 4614 such as a resin. The secondelectrode layer 4031 corresponds to a common electrode (a counterelectrode). The second electrode layer 4031 is electrically connected toa common potential line.

In FIG. 6, FIG. 7, and FIG. 8, a flexible substrate as well as a glasssubstrate can be used as the first substrate 4001 and the secondsubstrate 4006. For example, a plastic substrate having alight-transmitting property or the like can be used. As plastic, afiberglass-reinforced plastics (FRP) plate, a polyvinyl fluoride (PVF)film, a polyester film, or an acrylic resin film can be used. Inaddition, a sheet with a structure in which an aluminum foil issandwiched between PVF films or polyester films can be used.

An insulating layer 4021 can be formed using an inorganic insulatingmaterial or an organic insulating material. Note that the insulatinglayer 4021 formed using a heat-resistant organic insulating materialsuch as an acrylic resin, polyimide, a benzocyclobutene resin,polyamide, or an epoxy resin is preferably used as a planarizationinsulating film. Other than such organic insulating materials, it ispossible to use a low-dielectric constant material (a low-k material), asiloxane-based resin, phosphosilicate glass (PSG), borophosphosilicateglass (BPSG), or the like. The insulating layer may be formed bystacking a plurality of insulating films formed using any of thesematerials.

There is no particular limitation on the method for forming theinsulating layer 4021, and the following method can be used depending onthe material: a sputtering method, a spin coating method, a dippingmethod, spray coating, a droplet discharge method (such as an inkjetmethod, screen printing, or offset printing), roll coating, curtaincoating, knife coating, or the like.

The display device displays an image by transmitting light from a lightsource or a display element. Therefore, the substrate and the thin filmssuch as the insulating film and the conductive film provided for thepixel portion where light is transmitted have light-transmittingproperties with respect to light in the visible-light wavelength range.

The first electrode layer and the second electrode layer (each of whichis also referred to as a pixel electrode layer, a common electrodelayer, a counter electrode layer, or the like) for applying voltage tothe display element may have light-transmitting properties orlight-reflecting properties, which depends on the direction in whichlight is extracted, the position where the electrode layer is provided,and the pattern structure of the electrode layer.

The first electrode layer 4030 and the second electrode layer 4031 canbe formed using a light-transmitting conductive material such as indiumoxide including tungsten oxide, indium zinc oxide including tungstenoxide, indium oxide including titanium oxide, indium tin oxide includingtitanium oxide, indium tin oxide (hereinafter referred to as ITO),indium zinc oxide, or indium tin oxide to which silicon oxide is added.

Alternatively, the first electrode layer 4030 and the second electrodelayer 4031 can be formed using one or more kinds of materials selectedfrom metals such as tungsten (W), molybdenum (Mo), zirconium (Zr),hafnium (Hf), vanadium (V), niobium (Nb), tantalum (Ta), chromium (Cr),cobalt (Co), nickel (Ni), titanium (Ti), platinum (Pt), aluminum (Al),copper (Cu), and silver (Ag); alloys of these metals; and nitrides ofthese metals.

A conductive composition including a conductive high molecular weightmolecule (also referred to as a conductive polymer) can be used for thefirst electrode layer 4030 and the second electrode layer 4031. As theconductive high molecular weight molecule, a so-called π-electronconjugated conductive polymer can be used. For example, polyaniline or aderivative thereof, polypyrrole or a derivative thereof, polythiopheneor a derivative thereof, and a copolymer of two or more of aniline,pyrrole, and thiophene or a derivative thereof can be given.

Since the transistor is easily broken owing to static electricity or thelike, a protection circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

As described above, by using the transistor described in Embodiment 1, asemiconductor device having high reliability can be provided. Note thatthe transistor described in Embodiment 1 can be applied to not onlysemiconductor devices having the display functions described above butalso semiconductor devices having a variety of functions, such as apower device which is mounted on a power supply circuit, a semiconductorintegrated circuit such as an LSI, and a semiconductor device having animage sensor function of reading information of an object.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Embodiment 3

A semiconductor device disclosed in this specification can be applied toa variety of electronic devices (including game machines). Examples ofelectronic devices are a television set (also referred to as atelevision or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone (also referred to as a cellularphone or a mobile phone device), a portable game machine, a portableinformation terminal, an audio reproducing device, a large-sized gamemachine such as a pachinko machine, and the like. Examples of anelectronic device including the liquid crystal display device describedin the above embodiment will be described.

FIG. 9A illustrates a laptop personal computer which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying the semiconductor device described in Embodiment 1or 2, the laptop personal computer can have high reliability.

FIG. 9B illustrates a portable information terminal (PDA) which includesa display portion 3023, an external interface 3025, operation buttons3024, and the like in a main body 3021. A stylus 3022 is provided as anaccessory for operation. By applying the semiconductor device describedin Embodiment 1 or 2, the portable information terminal (PDA) can havehigher reliability.

FIG. 9C illustrates an example of an electronic book reader. Forexample, an electronic book reader 2700 includes two housings, a housing2701 and a housing 2703. The housing 2701 and the housing 2703 arecombined with a hinge 2711 so that the electronic book reader 2700 canbe opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can be used like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, a display portion onthe right side (the display portion 2705 in FIG. 9C) can display textand a display portion on the left side (the display portion 2707 in FIG.9C) can display graphics. By applying the semiconductor device describedin Embodiment 1 or 2, the electronic book reader 2700 can have highreliability.

Further, FIG. 9C illustrates an example in which the housing 2701 isprovided with an operation portion and the like. For example, thehousing 2701 is provided with a power switch 2721, operation keys 2723,a speaker 2725, and the like. With the operation keys 2723, pages can beturned. Note that a keyboard, a pointing device, or the like may also beprovided on the surface of the housing, on which the display portion isprovided. Furthermore, an external connection terminal (such as anearphone terminal or a USB terminal), a recording medium insertionportion, and the like may be provided on the back surface or the sidesurface of the housing. Moreover, the electronic book reader 2700 mayhave a function of an electronic dictionary.

The electronic book reader 2700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired book data or the like can be purchased anddownloaded from an electronic book server.

FIG. 9D illustrates a mobile phone which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 having a functionof charging the portable information terminal, an external memory slot2811, and the like. Further, an antenna is incorporated in the housing2801. By applying the semiconductor device described in Embodiment 1 or2, the mobile phone can have high reliability.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 that is displayed as images isillustrated by dashed lines in FIG. 9D. Note that a boosting circuit bywhich voltage output from the solar cell 2810 is raised to besufficiently high for each circuit is also provided.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the mobile phone isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Further, thehousings 2800 and 2801 in a state where they are developed asillustrated in FIG. 9D can shift by sliding so that one is lapped overthe other; therefore, the size of the mobile phone can be reduced, whichmakes the mobile phone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer or the like are possible.Moreover, a larger amount of data can be saved and moved by inserting arecording medium to the external memory slot 2811.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 9E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Byapplying the semiconductor device described in Embodiment 1 or 2, thedigital video camera can have high reliability.

FIG. 9F illustrates an example of a television set. In a television set9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. By applying the semiconductor devicedescribed in Embodiment 1 or 2, the television set 9600 can have highreliability.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, a general televisionbroadcast can be received. Moreover, when the television set isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) data communication can beperformed.

The structures, methods, and the like described in this embodiment canbe combined as appropriate with any of the structures, methods, and thelike described in the other embodiments.

Example 1

In this example, a contact hole formed in the following manner wasevaluated: insulating films were formed so that a first silicon oxide(SiO_(x)) film, a gallium oxide (GaO_(x)) film, and a second siliconoxide (SiO_(x)) film were sequentially stacked in a manner similar tothat of Embodiment 1, and dry etching was performed on the insulatingfilms with the use of a photolithography step.

Firstly, a manufacturing process of the insulating films used in thisexample, in which the first silicon oxide film, the gallium oxide film,and the second silicon oxide film were sequentially stacked, isdescribed.

First, the first silicon oxide film was formed over a tungsten substrateso as to have a thickness of 100 nm Here, the first silicon oxide filmwas formed by a sputtering method under conditions where the flow rateof an argon gas was 25 sccm, the flow rate of an oxygen gas was 25 sccm,the power of an RF power source was 2 kW, the pressure was 0.4 Pa, andthe temperature was 100° C.

Next, the gallium oxide film was formed over and in contact with thefirst silicon oxide film so as to have a thickness of 100 nm Here,gallium oxide was deposited by a sputtering method under conditionswhere the flow rate of an argon gas was 10.5 sccm, the flow rate of anoxygen gas was 4.5 sccm, the power of the RF power source was 200 W, thepressure was 0.4 Pa, and the temperature was room temperature.

Next, the second silicon oxide film was formed over the gallium oxidefilm so as to have a thickness of 300 nm Here, the second silicon oxidefilm was formed by a sputtering method under conditions where the flowrate of an argon gas was 40 sccm, the flow rate of an oxygen gas was 10sccm, the power of the RF power source was 1.5 kW, the pressure was 0.4Pa, and the temperature was 100° C. Through the above steps, theinsulating films in which the first silicon oxide film, the galliumoxide film, and the second silicon oxide film were sequentially stackedwere formed.

Then, dry etching was performed on the insulating films, so that acontact hole was formed. Here, a contact hole was formed by forming aresist mask over the second silicon oxide film, performing dry etchingon the insulating films, and removing the resist mask by only ashingusing oxygen plasma, so that Sample A was formed. A contact hole wasformed by forming a resist mask over the second silicon oxide film,performing rinse treatment, performing dry etching on the insulatingfilms, and removing the resist mask by ashing using oxygen plasma andtreatment using a resist stripper solution, so that Sample B was formed.

A manufacturing process of Sample A is described in detail. First, theresist mask was formed over the second silicon oxide film by aphotolithography method. Here, in the manufacture of Sample A, rinsetreatment was not performed when a resist pattern was formed.

Next, with the use of the resist mask, dry etching was performed on theinsulating films having the structure in which the first silicon oxidefilm, the gallium oxide film, and the second silicon oxide film weresequentially stacked, so that the contact hole was formed. Here, the dryetching was performed by an ICP etching method. Etching conditions wereset as follows: the flow rate of a trifluoromethan gas was 7.5 sccm; theflow rate of a helium gas was 142.5 sccm; the power applied to acoil-shaped electrode was 475 W; the power applied to an electrode onthe substrate side was 300 W; the pressure was 5.5 Pa, and thetemperature of a lower electrode was 70° C.

Then, the resist mask formed over the second silicon oxide film wasremoved by only ashing using oxygen plasma. Here, conditions of theashing treatment using oxygen plasma were set as follows: the flow rateof an oxygen gas was 300 sccm; the power of the RF power source was 1800W; the pressure was 66.5 Pa; and the treatment time was 180 seconds.Through the above steps, Sample A was manufactured where the contacthole was formed in the insulating films in which the first silicon oxidefilm, the gallium oxide film, and the second silicon oxide film weresequentially stacked.

Next, a manufacturing process of Sample B is described in detail. Adifference between the manufacturing processes of Sample A and Sample Bis in that rinse treatment was performed in the manufacturing process ofSample B when a resist pattern was formed. In addition, in themanufacturing process of Sample B, the resist mask was removed by ashingusing oxygen plasma and treatment using a resist stripper solution.Conditions of the ashing treatment were set as follows: the flow rate ofan oxygen gas was 100 sccm; the power of the RF power source was 200 W;the pressure was 66.5 Pa; and the treatment time was 300 seconds. As theresist stripper solution, a stripping agent N-300 (produced by Nagase &Co., Ltd.) was used. The other steps of manufacturing Sample B wereperformed by a method similar to that of the manufacturing process ofSample A.

FIGS. 10A and 10B are SEM images of Sample A and Sample B, respectively.When FIGS. 10A and 10B are compared with each other, a surface of thesecond silicon oxide film is kept clean in Sample A, whereas a residueof the resist mask, a reaction product of the residue, and the likeremain on a surface of the second silicon oxide film in Sample B.

FIGS. 11A and 11B are respective enlarged SEM images of cross-sectionalportions of the insulating films in the SEM images of FIGS. 10A and 10B.From comparison between FIGS. 11A and 11B, it is found that a sidewallof the contact hole projects in the gallium oxide film portion to form astepped shape in Sample A, whereas a sidewall of the contact hole isdeeply dissolved in the gallium oxide film portion to form a depressionin Sample B. Thus, the gallium oxide film in the sidewall portion, whichis needed for the contact hole, is also removed in Sample B.

As described above, a region which is in a gallium oxide film and isneeded in design might be dissolved by wet treatment such as rinsetreatment or treatment using a resist stripper solution as in Sample B.Therefore, by employing a manufacturing process in which a gallium oxidefilm is not directly subjected to wet treatment in a step of forming acontact hole in an insulating film including gallium oxide as in SampleA, a possibility that a region which is in the gallium oxide film and isneeded in design is dissolved can be reduced.

EXPLANATION OF REFERENCES

110: transistor, 200: substrate, 202: gate electrode, 204: insulatingfilm, 206: oxide semiconductor film, 208 a: source electrode, 208 b:drain electrode, 210: insulating film, 212: insulating film, 214: resistmask, 216: wiring, 218: contact hole, 222: wiring, 224: contact hole,301: target, 302: target, 310: transistor, 400: substrate, 402: gateelectrode, 406: oxide semiconductor film, 408 a: source electrode, 408b: drain electrode, 410: insulating film, 412: insulating film, 414:resist mask, 416: wiring, 418: contact hole, 2700: electronic bookreader, 2701: housing, 2703: housing, 2705: display portion, 2707:display portion, 2711: hinge, 2721: power switch, 2723: operation key,2725: speaker, 2800: housing, 2801: housing, 2802: display panel, 2803:speaker, 2804: microphone, 2805: operation key, 2806: pointing device,2807: camera lens, 2808: external connection terminal, 2810: solar cell,2811: external memory slot, 3001: main body, 3002: housing, 3003:display portion, 3004: keyboard, 3021: main body, 3022: stylus, 3023:display portion, 3024: operation button, 3025: external interface, 3051:main body, 3053: eyepiece, 3054: operation switch, 3055: display portionB, 3056: battery, 3057: display portion A, 4001: substrate, 4002: pixelportion, 4003: signal line driver circuit, 4004: scan line drivercircuit, 4005: sealant, 4006: substrate, 4008: liquid crystal layer,4010: transistor, 4011: transistor, 4013: liquid crystal element, 4015:connection terminal electrode, 4016: terminal electrode, 4018: FPC, 4018a: FPC, 4018 b: FPC, 4019: anisotropic conductive film, 4021: insulatinglayer, 4030: electrode layer, 4031: electrode layer, 4032: insulatingfilm, 4033: insulating film, 4510: partition wall, 4511:electroluminescent layer, 4513: light-emitting element, 4514: filler,4612: cavity, 4613: spherical particle, 4614: filler, 4615 a: blackregion, 4615 b: white region, 9600: television set, 9601: housing, 9603:display portion, and 9605: stand. This application is based on JapanesePatent Application serial no. 2010-139715 filed with Japan Patent Officeon Jun. 18, 2010, the entire contents of which are hereby incorporatedby reference.

The invention claimed is:
 1. A semiconductor device comprising: an oxidesemiconductor layer comprising a channel formation region over aninsulating surface; a source electrode and a drain electrode connectedto the oxide semiconductor layer; a first layer over and in contact withthe source electrode, the drain electrode, and the channel formationregion; a second layer over the first layer; and a wiring over thesecond layer; wherein the first layer comprises gallium and oxygen,wherein the wiring is connected to the source electrode or the drainelectrode through a contact hole in the first layer and the secondlayer, and wherein a diameter of the contact hole in the first layer issmaller than a diameter of the contact hole in the second layer.
 2. Thesemiconductor device according to claim 1, wherein the first layercomprises aluminum oxide.
 3. The semiconductor device according to claim1, wherein the first layer is an insulating layer comprising galliumoxide.
 4. The semiconductor device according to claim 1, wherein thesecond layer is an insulating layer comprising silicon and at least oneof oxygen and nitrogen.
 5. The semiconductor device according to claim1, wherein the second layer is over and in contact with the first layer.6. The semiconductor device according to claim 1, further comprising agate electrode below the oxide semiconductor layer.
 7. The semiconductordevice according to claim 1, further comprising a gate electrode overthe oxide semiconductor layer.
 8. The semiconductor device according toclaim 1, wherein the contact hole includes a stepped shape.
 9. Thesemiconductor device according to claim 1, wherein the oxidesemiconductor layer comprises gallium.
 10. A semiconductor devicecomprising: an oxide semiconductor layer comprising a channel formationregion over an insulating surface, the oxide semiconductor layercomprising indium and zinc; a source electrode and a drain electrodeconnected to the oxide semiconductor layer; a first layer over and incontact with the source electrode, the drain electrode, and the channelformation region; a gate insulating layer over the first layer; a gateelectrode over the gate insulating layer; a second layer over the gateelectrode; and a wiring over the second layer; wherein the first layercomprises gallium and oxygen, and wherein the wiring is connected to thesource electrode or the drain electrode through a contact hole in thefirst layer and the second layer.
 11. The semiconductor device accordingto claim 10, wherein the first layer comprises aluminum oxide.
 12. Thesemiconductor device according to claim 10, wherein the first layer isan insulating layer comprising gallium oxide.
 13. The semiconductordevice according to claim 10, wherein the second layer is an insulatinglayer comprising silicon and at least one of oxygen and nitrogen. 14.The semiconductor device according to claim 10, wherein the second layeris over and in contact with the first layer.
 15. The semiconductordevice according to claim 10, wherein the contact hole includes astepped shape.
 16. The semiconductor device according to claim 10,wherein the oxide semiconductor layer comprises gallium.